Magnetic flux flows along the path of least reluctance, i.e. the path of greatest permeability. As a result, high frequency integrated magnetic devices are designed and fabricated such that the AC magnetic field is applied along the direction of greatest permeability. This design choice requires that the hard-axis of the magnetic core be oriented parallel to the AC field (i.e. the easy-axis perpendicular to the field), resulting in maximum quality factor and magnetic coupling efficiency. A drawback of current fabrication methods is that they typically permit only a single, uniform field to be applied during deposition. Under such deposition conditions, closed magnetic loops cannot be easily fabricated for maximum efficiency since only the regions perpendicular to the applied field emerge with induced hard-axis permeability.
Previous work has been done on the control of permeability post-fabrication, however the application has been mostly focused on frequency and inductance tuning of magnetic devices during operation. Such work includes topologies directed to an electromagnetic conversion element and variable inductance that utilize a sandwiched magnetic and piezoelectric beam structure, and a general inductance device in which the permeability and inductance are varied using a comb-like electrode on a piezoelectric/magnetic composite structure.
In-situ tuning of inductors using a bias current in an integrated tunable magnetic RF inductor has been attempted. Other variations include processing and application of magnetoelastic thin films in high-frequency devices in electrostatically tunable magnetoelectric inductors with large inductance tunability.
Other work has been directed to control of permeability, however it continues to be limited to linear magnetic thin-film structures. In the prior work, the permeability of a radio frequency (RF) magnetic device was varied while seated on top of and controlled by a suspended, mobile piezoelectric material.
For thin-film magnetic cores forming closed magnetic loops, a single applied field during deposition is not sufficient to induce universal hard-axis permeability throughout the loop. For example, in a toroid, regions where the magnetic core is parallel to the applied field during deposition obtain local easy-axis permeability, which is effectively zero for high-frequency alternating currents and therefore does not contribute to the performance of the device.
What is needed is, a method for fabricating thin-film magnetic cores with multi-axial anisotropy, resulting in a composite multiferroic device that induces a local magnetic anisotropy in the desired direction post-deposition.